Parameter
Scenario 1
Scenario 2
Scenario 3
Bottom breach width (m)
6.9
6.2
8.8
Breach slopes (H:1V)
0.6
0.6
0.6
Formation time (hr)
0.23
0.23
0.25
4.3 Outflow hydrographs Outflow hydrographs were derived following standard breach routing equations by Fread [23] implemented in the software HEC-HMS (version 4.12). Hydrograph estimations considered the unique stage-storage relationship of the outflow failure surface and associated decant pond, as well as the breach parameters presented in Table 4.3. Table 4.4 summarises the resulting peak flows for each of the assessed scenarios. Derived breach hydrographs are provided in Figure 4.6.
Table 4.4
STSF breach hydrographs
Peak outflow (m 3 /s)
Scenario
1 (Stage 1 Sunny Day Failure)
13,234
2 (Stage 3 Sunny Day Failure)
24,033
3 (Stage 5 Sunny Day Failure)
31,239
Figure 4.6: Outflow hydrographs for Sandstone Gully TSF
The peak outflows for all scenarios presented in Table 4.4 have been validated against published research and studies on available historic dam failures and peak discharge. Historical peak discharges are plotted against the calculated dam height in Figure 4.7.
Project No PS213278 Sandstone Gully TSF Dam Break and Consequence Category Assessments Mount Morgan Gold Mine Heritage Minerals
WSP January 2025 Page 19
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